It reported that four Americans were detained on Monday and another five in the following days, along with 14 Saudis, three Yemenis, two Syrians, an Indonesian, a Filipino, a United Arab Emirates citizen, a Palestinian and a citizen of Kazakhstan.

The American embassy did not immediately respond to a request for comment.

Saudi Arabia in 2014 declared Islamic State a terrorist organization and has detained hundreds of its supporters. The group, which controls territory in Iraq and Syria, has staged a series of attacks in the kingdom.

On Friday an attack at a Shi‘ite Muslim mosque in Saudi Arabia’s al-Ahsa district in Eastern Province killed four people and injured 18, the latest in a string of attacks claimed by Sunni jihadists that have left over 50 dead in the past year.

The website of the Interior Ministry’s militant rehabilitation center listed four U.S. citizens as having been detained on Jan 25 and four more over the previous three months. It did not yet list any more recent detentions.

The Interior Ministry spokesman pointed Reuters toward that website, which gives information on all people detained as militant suspects, but gave no further comment.

The ministry on Saturday identified one of the attackers in al-Ahsa as 22-year-old Abdulrahman al-Tuwaijri, a Saudi citizen, who detonated an explosive vest outside the Imam Rida mosque in the Mahasen district of Hofuf in al-Ahsa.

A 27-year-old was also arrested wearing an explosive vest and carrying hand grenades after members of the mosque’s congregation seized him after he fired shots at them during the attack, the ministry said.

Attacks by supporters of Islamic State in Saudi Arabia include two bombings and two mass shootings at Shi‘ite mosques. A mosque used by Sunni security services was also bombed

The Saudi clergy have denounced the group as “kharijites”, an early Islamic sect reviled by Muslims for its extreme ideology.